The present invention provides a number of modified dyes, such as xanthene dyes, including rhodamines, rhodols and fluoresceins that are substituted with one or two phosphonate groups that are either zwitterionic or a protected form thereof. The dyes of the invention, including chemically reactive dyes and dye-conjugates are useful as fluorescent probes, particularly in biological samples.
Fluorescent dyes are known to be particularly useful in biological applications where a highly sensitive detection reagent is required. Fluorescent dyes are used to label both visible color and fluorescence to other materials. The dyes of this invention are phosphonate derivatives of xanthene-based dyes that are typically fluorescein, rhodamine or rhodol derivatives

“Fluorescein” dyes include derivatives of 3H-xanthen-6-ol-3-one that are typically substituted at the 9-position by a 2-carboxyphenyl group. “Rhodamine” dyes include derivatives of 6-amino-3H-xanthen-3-imine that are typically substituted at the 9-position by a 2-carboxyphenyl group. “Rhodol” dyes include derivatives of 6-amino-3H-xanthen-3-one that are typically substituted at the 9-position by a 2-carboxyphenyl group. Fluoresceins, rhodamines and rhodols are typically substituted by a derivative capable of forming a 5- or 6-membered lactone or lactam ring. For example in the case of fluorescein the spirolactone form of the dye has the following structure:

Many commercial fluorescent dyes have a polycyclic aromatic nature and are hydrophobic. Those molecules are also prone to minimize exposure to any hydrophilic environment through interactions with nearby hydrophobic surfaces and residues. These interactions include dye-dye interaction and dye-biomolecule (e.g. proteins, lipids, oligonucleoties) interactions. Hydrophobic interactions can cause substantial quenching effect for fluorescent dyes (see for example Randolph, J. B.; Waggoner, A. S. Nucleic Acids Res. 1997, 25(14), 2923-2929 and references cited therein). One method to overcome this problem is to improve the hydrophilic character of the dye by, for example, introducing a sulfonate substituent into the dye molecule (sulfonated carbocyanine dyes are disclosed in U.S. Pat. No. 5,268,486 and sulfonated xanthene dyes are disclosed in U.S. Pat. No. 6,130,101).
The present invention describes dyes, including fluorescein, rhodol and rhodamine dyes that are substituted by at least one phosphonate moiety. The phosphonate-substituted dyes of the invention possess considerable advantages over their unsubstituted hydrophobic analogs. Being ionized, the phosphonate group brings additional hydrophilicity to the dye molecules, increasing their solubility in aqueous solutions. Importantly, the quantum yield of the new dyes is not decreased by introduction of a phosphonate moiety. In contrast to the sulfonated dyes, the dyes of the invention are compatible with the synthetic conditions used on commercial oligonucleotide synthesizers.